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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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ET rates between both systems.
Osuka et al. have prepared another type of hydrogen-bonded complex
between porphyrin 126 and 1,8:4,5-naphthalene-tetracarboxamide.140
Porphyrin 126, having
2,6-diaminopyridine as a binding site, interacts with the diimide via
three-point hydrogen bonds with an association constant of 1.3 x 105 M -
1 in CftD6. Stern-Volmer plots for the fluorescence quenching of 126 with
the diimide reveal efficient quenching due to stable complex formation in
the ground state. Furthermore, the picosecond transient absorption
spectra of 126 in the presence of the diimide revealed significant
amounts of porphyrin cation radical species at 580-680 nm with a broad
absorption within 20 ps, although they did not discuss the kinetic data
in detail.
2. Divergently Linked Donor-Acceptor Pairings Formed by Hydrogen Bonding
The first example of a sideways-linked porphyrin-quinone complex was
presented by Harriman, Sessler and coworker in 1992.141 Porphyrin 127 has
a guanine subunit bound to one of four meso positions, while p-
benzoquinone as an electron acceptor is linked with a cytosine subunit to
46 / Porphyrins and Metalloporphyrins as Receptor Models
315
N-H....0
126-diimide
form a Waison-Crick hydrogen-bonding interaction. The association
constant derived from a 'H NMR study is
3.1 x lO'M 1 in CDiCb. Upon addition of the quinone. the fluorescence of
the zinc porphyrin was quenched and its decay profile became biphasic
with two exponential components of lifetimes. -1.5 ns and rs=
0.94 ns,
respectively. Both lifetimes did not depend on the concentration of the
quinone, while the relative contribution of the shorter component
increased as the quinone concentration was increased, suggesting a
porphyrin-quinone complex formation. The rate constant of forward ET from
pholoexcited porphyrin to quinone was estimated to be 4.2 x 10ss '.
The subsequent similar ensemble by use of Watson-Crick base-pairing
interaction was reported by Sessler et al.142 Compared to the previous
system, porphyrin 128 and the corresponding cytidine-siibstituted quinone
may be more eonforrnationally rigid, so this system could be used to
better evaluate ET within a noncovalently linked complex.
The second generation system was also associated via triple hydrogen
bonding. Fluorescence lifetimes in the presence of the quinone were found
to be t, = 1.8 ns and Ús = 0.74 ns, and the fractional contribution of
the shorter-lived component leads to the association constant of
8.99xlO,M 1 in CFLCL. Furthermore, the rate constant within the complex
was estimated to be 8 x 10s s ', where the edge-to-edge distance of the
porphyrin-quinone ring is about 14 A and AG is - 0.5 eV. However,
picosecond laser flash photolysis has never shown the significant
transient absorption assigned as the porphyrin ˇ-cation radical, since
the charge recombination process could be fast. On the other hand,
Levanon, Sessler and coworkers showed direct evidence of photoinduced ET
in the fluid nematic phase of system 128 by use of time-resolved electron
paramagnedc resonance spectroscopy with laser excitation at 532 nm.141
Sessler and coworkers later presented pholoinduced ET in a
noncovalently linked porphyrin-quinone ensemble through a calixarene
subunit.144 Probably, the quinone carbonyl oxygen associated with the
hydroxy groups of a calixarene group pendant from the zinc porphyrin 129
with an association constant of 40 M_l. Stern-Volmer plots were found to
be curved upward, suggesting that static and dynamic quenching occurred
between the porphyrin and quinone. The photoinduced ET within the complex
was also estimated from fluorescence lifetime measurements (kci = 8.0 x
108 s - 1) through bond and/or through space.
A rigid chlorin-diimide complex via hydrogen-bonding interactions was
also reported by BSessler and coworkers. This model is a eoplanar
noncovalent ET system composed of modified chlorin 130 and naphthalene
diimide with the uracil-type hydrogen-bonding recognition site.l4S The
association constant, determined by 'H NMR, is
316
Ogoshi et al.
RO OR
R = -SilV^Bu*
128-quinone
129-BQ
Ar = 3,5-di-ferf-butylphenyl
364 M_1 in CDCI3. Time-resolved fluorescence spectroscopy of this system
showed two fluorescence lifetimes, at
13.2 ns and 1.2 ns, which were attributed to free and diimide-bound
porphyrins, respectively. From these values, the rate constant from
photoexcited 130 to the diimide acceptor was determined to be 7.6 x
108s~' (AG =
- 457 mV; edge-to-edge distance is about 7 A). However, the reduced
diimide, radical anion species, could not be monitored by laser flash
photolysis.
A different approach for the construction of divergently linked donor-
acceptor complexes was reported by D'Souza.146 Hydroquinone-appended
porphyrin 131 as a photodonor was prepared to associate with a quinone
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